The Ministry of Health confirmed on Thursday (18/12) four cases in Brazil of the K subclade of Influenza A (H3N2), a variant popularly referred to as flu K. According to the ministry, one case was recorded in the state of Pará and is associated with international travel, while three cases were identified in Mato Grosso do Sul and are still under investigation to determine their origin. In an official statement, the ministry clarified that the K subclade represents a genetic variation of Influenza A (H3N2) and does not constitute a new virus. This variant is already circulating in North America, Europe, and Asia.
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Presence in Brazil
The influenza-like illness surveillance report released on December 12 had already confirmed the presence of the K subclade in Brazil, although without providing detailed information at the time. The same document also identified the circulation of another genetic variation of the virus, the J.2.4 subclade of Influenza A (H3N2).
WHO Alert for 2026
Last week, the World Health Organization (WHO) issued an alert regarding the increased global circulation of the influenza virus since October. According to the WHO, most of the recent cases are linked to the K subclade (J.2.4.1) of Influenza A (H3N2). The organization noted that the rise in cases coincides with the onset of winter in the Northern Hemisphere and a broader increase in acute respiratory infections caused by seasonal viruses.
Earlier in Brazil
As a result of this global increase, the Pan American Health Organization (PAHO) warned that the influenza season may begin earlier than usual in the Southern Hemisphere, including Brazil, in 2026. In Brazil, peak virus circulation typically occurs between June and August, but health authorities are monitoring the possibility of earlier transmission.
Vaccine
In early December, Brazil’s National Health Surveillance Agency (Anvisa) defined the strains that will compose the influenza vaccines for use in Brazil in 2026. The Influenza A (H3N2) subtype is included, although the specific K subclade is not. The WHO emphasizes that vaccination remains essential to prevent severe flu outcomes, particularly among high-risk groups. In Brazil’s public health system (SUS), influenza vaccination is offered primarily to vulnerable populations, including children aged 6 months to under 6 years, pregnant women, and the elderly.
Main Symptoms
The main symptoms associated with influenza K are similar to those of seasonal flu and include high fever (above 38°C), chills, headache, muscle and joint pain, intense fatigue and prostration, dry cough, sore throat, runny or congested nose, and general malaise. According to the Ministry of Health, symptom intensity may vary depending on age. Children may develop very high fever and swollen lymph nodes in the neck, and in some cases may experience respiratory complications such as bronchitis or bronchiolitis, as well as gastrointestinal symptoms. Among the elderly, fever is almost always present, sometimes without other symptoms, and body temperature generally does not reach very high levels.
Sources: A Folha de SP.
